First of all, make sure that the computer browser, workstation and server services is started, followed by ensuring that all services configured to start automatically are started. The next thing that you should do, and some might suggest that you should do this first, check your event logs — particularly application and system — for errors and warnings. Cross-reference any event IDs on the Microsoft Knowledge Base and EventID.net.
You will also want to perform a full scan of the server for computer viruses and other malware. There are a lot of free malware scanning and removal tools that you can use such as Spybot Search & Destroy, Lavasoft Ad-Aware, Microsoft and Windows Defender only to name a few. Make sure that you are using the latest virus scanning engine and definitions.