A few things you can do are:
- recycle the app pool on the website
- query the
cms_eventlog table for the top 100 events and see if anything unusual is showing up
- check the event log of the machine the website is running on and see if there are any
- validate your connection string is correct
- ensure your site builds properly in Visual Studio
There are a good dozen other things you can look at as well but typically those things are where I start.