1- Check Eventviewer

2- Check disk free space

3- Try login with SQLCMD using [sysadmin] windows login or sa if it works

4- Read database Error Log, what is going on the server.

5- Check history to see what is changed/modified or ran over last moments.

There is a list going on. See if this helps to have some clue on issue and you can resolve.

Shamshad Ali