There is overwhelming evidence that suggest Stalin anticipated a war with Nazi Germany. He knew that in 1939 the USSR was completely unprepared to fight a war with Germany, and signed the Molotov-Rippentrop pact to buy the time needed. This let the Nazi's not have to worry about the second front when they split up Poland. The USSR upped their military production consistently from 1939 to 1941. Stalin told Beria, he had fooled Hitler.

There were many attempts on Stalin, including from Germany during the war, but if you are asking if there was a mass conspiracy ala Operation Valkyrie, then the answer is no. Some do think that Stalin was ultimately murdered in 1953, but there is not a lot of evidence to suggest anything more sinister than delayed treatment for a stroke. Per the autopsy reports, Stalin was in pretty rough shape, with a fattly liver, aterial blockage, and high blood pressure.

There're no plots that historians are certain of. The inner circle Stalin had constructed in the 1930s made revolt against his authority impossible. This was as much a mental construct as a physical one; Stalin in his interactions with his closest allies and subordinates combined friendliness and comfortably course behavior with the constant threat of torture and death of them or their family/friends. Mikhail Kalinin's wife was imprisoned in the Gulag in 1938, Lazar Kaganovich's former coworkers were imprisoned or executed, Vyacheslav Molotov's wife was nearly imprisoned, Anastas Mikoyan from Armenia had to oversee the purge of its entire Politburo and Party, and Nikolai Yezhov broke down due to the constant terror he experienced and was executed. Everyone lost close friends during the Purge and experienced numerous menacing comments from Stalin seemingly at random. Even as lavish parties were thrown they lived a life of fear. Even as Yagoda, then Yezhov, and then Beria worked to stamp out real or imagined threats they and those around them were never safe.

The lives and fortunes of the men that served Stalin were quite literally held in his hands. If he chose to he could, and sometimes did, have them, their friends, and their family destroyed; and this was made explicitly clear on many an occasion. It's quite obvious why even in his brief moment of weakness days after Barbarossa they still came asking him to lead the GKO (State Defense Committee). They lived in such a constant state of terror and confusion that acting against him was inconceivable. Everything, including the lives of their closest friends and family, was secondary to their work and Stalin's will.

That's not to make you pity them mind; these were men that willingly and fervently assisted in Stalin's many crimes. But I hope it does give an idea as to why, beyond the physical obstacles to organizing a coup, it couldn't even have begun to form in their minds.

Sources:

Master of the House: Stalin and His Inner Circle by Oleg Khlevniuk

Stalin: Court of the Red Tsar by Simon Montefiore

Stalin and his Hangmen: The Tyrant and Those who Killed for Him by Donald Rayfield

Stalin died in March 1953.

The way the argument about whether or not Stalin was murdered goes something like this:

Khrushchev "writes,"* in his memoirs, that Beria told him that he, Beria "saved" them all from Stalin. What does that mean? Well, it was fairly well known, or at least felt by high party officials at the time that Stalin was preparing for a "right-ward" purge. Stalin's modus operandi throughout most of his reign from the 20's onward was to engage in "purges" of political opponents, at time purging people whose politics were to the "left" (as Bolshevik politics viewed such things) and other times to the "right." A left-ward example would be the purge of the "Trotskyist Left" including Trotsky himself and Zinoviev. A right-ward purge would be the purge of Bukharin and others a few years after the anti-Trot purge.

It was believed, again, by Khruschev and Molotov, that the "right" people to be targeted would be Zhukov, Molotov, Beria, Khrushchev, and possibly Bulganin, and Voroshilov.

So, now that we have the context that many prominent Politburo members in 1953 felt that Stalin was preparing a "right-ward" purge, we should examine Beria's comments to Khrushchev (and separately to Molotov), that he "saved" them. Khrushchev and Molotov both interpreted this as a claim that Beria had killed Stalin. Recent articles have examined this claim. See: NY Times Article on study and Study.

[edit to add]The Study alleges that Stalin was poisoned with what is essentially rat poison. Certainly Beria, as head of the NKVD would have been in a position to do so, if he wanted to. So that was Beria's claim: he killed Stalin, saving them from the right-ward purge. Of course, he made these statements in the context of the post-Stalin's-death power struggle where Khruschhev, Malenkov, and Bulganin eventually formed a triumvirate. So Beria was trying to curry favor with the, quite unsuccessfully, as he was executed.

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Sources: Khrushchev Remembers.

Molotov memoirs.

Both from my recollection of reading them.

*I say "writes" because there's a great deal of evidence that he had a pretty low level of literacy and that it was dictated. This had led to some historicism-style speculation about the degree of editing that was done.