Hi there, if it is permitted, here's an answer from a previous thread asking the same question. Unfortunately the account has been deleted so I can't credit the user.
The general election of 5th July, 1945, resulted in what many people at the time considered a surprise victory for Atlee and the Labour Party. Churchill was immensely personally popular, and possibly partially because of this the Conservative Party failed to mount a serious campaign. There was also some degree of public feeling that "new blood" was needed to shape post-war Britain, and after depression followed by wartime austerity Labour's platform was appealing to the public and the returning soldiers (including a national health service, full employment… in effect a true welfare state). Conservatives, on the other hand, didn't really propose serious domestic policy initiatives and were relying on the "we're the party of Winston" approach, weakening them significantly going into the election.
It is true the war was still ongoing at this time… however, the war in Europe had been over two months, and the defeat of Japan was by this point a virtual certainty. People were beginning to come out of a wartime mindset and consider the afterwar period.
An additional consideration is that the Conservatives had been in power during the 1930's… and so Labour could paint them as the party of appeasement, the party that failed to re-arm, etc., in anticipation of the coming war… the party that had failed at peace. And with another peace coming, this was an important factor to voters, who didn't want a Third World War. To some degree as well, Churchill, while popular, was portrayed as old fashioned and out of touch… he was in his 70's by this point, and had been actively involved in public life since the Victorian period.
Churchill again became Prime Minister from 1951-1955, when the conservatives returned to power. By 1955 he had suffered a series of health issues (strokes), and effectively retired to be succeeded by Anthony Eden. During this second period in power he made an effort to retain control of the disintegrating British Empire, and mostly failed at doing so.
So to answer your question, Churchill was hugely popular, and Atlee's victory was a surprise, but the stage was set for it going into the election. I hope this answers your questions! Cheers!
Sources: William Manchester's "The Last Lion" Vol. I and Vol. II; Winston Churchill's "The Second World War"
If I can add to this, some political historians believed that Churchill/Conservatives lost due to his conservative nature, i.e. representing the then present, or the status quo. The argument goes that after the war, many Britons simply just wanted to move forward and modernise, hence a conservative (a traditionalist party in the UKs) defeat.
Just remember a few things. Firstly the UK is very diverse and the wealth gap would be even bigger after the war. There were people who voted conservative, but also people left poorer would see the appeal of the collectivism advocated by the Labour Party (e.g. NHS).
Secondly and perhaps most importantly, the UK went through what's called the 'National Government'; this is where all parties put traditional adversarial politics behind and come together to run the country together, for the good of the war effort (similar to a coalition government). What can we take from this? Well, in reality, while Churchill may have been perceived by history as a good leader, the conservatives never really fully ruled during the war. So it wasn't really the black and white case of not reelecting the party who 'won us the war'.
This fantastic source from the BBC details arguments from multiple historians: http://www.bbc.co.uk/history/worldwars/wwtwo/election_01.shtml
Edit: Please note the 'Wealth Gap' source although about modern Britain, details how big the wealth gap is after WWII:
"The last time the best-off took as big a share of all income as they do today was in 1940, two years before the publication of the Beveridge Report, which became the basis of the UK's welfare state after the Second World War."
A little late and not to curtail any further discussion, we have a a whole section about this in our FAQ