Cannot switch between users

We have an iMac running High Sierra 10.13.2.  It seems to run very slow anyway (which is why I never use it) but my main frustration at the moment is that I cannot switch to my desktop.  After I log in, it just spins forever until I have to turn the computer off.  what can I do I get to my desktop?  And, if anybody has any tips on how we can determine why and how it's gotten so slow, I'd love to hear them.

Thank you.

Has invited:

To reply to a question, please Login or registered