When I turn my computer on, the 4 user names appear and i usually just click the account picture that i wiwsh to use. none of the accounts have passwords. now when i turn the computer on the user names appear but i cannot click on them. the cursor does not change when i hover over them and i cannot click 'turn off computer' either. i have to restart it to get it to work. this used to only happen from time to time, however it is happening more frequently. what is going on?

Can you boot to the safe mode?
Probably you should repaire your system...

Try pressing Ctrl+Alt+Del repeatly in the Welcome screen.
You should get classic login screen. Now try to login.