Were people able to go to the bank and get loans or was that too capitalistic?

The banks were state-owned and did not loan money to private citizens. Instead of banks investing their clients money (as do banks in a capitalist system) money was invested into projects at the discretion of the state. In other words, money was put towards centrally planned projects. People could put their money into savings banks and get interest in return in a more or less familiar fashion.

If people weren't able to did loan-sharks and criminal organizations pick up the slack?

Not exactly. Because finance was all centrally controlled the black or grey market tended not work in terms of cash. The idea of blat, a term used to describe the network of people used to use/bend the system in their favor, come by rare/hard to come by/or even illegal goods is instead what tended to happen. Some of it was illegal some of it technically wasn't. It's somewhat comparable to "networking" as we might now describe it, but because of centrally controlled economy there is only so much use for actual money, favors, contacts, etc were incredibly valuable.

Alena Ledeneva's 1998 work on blat Russia's Economy of Favors is one of the more important works on this topic and is valuable to anyone who wants to understand the day to day Soviet Economy as experienced by actual people.