Are you running the latest version - 2015.3 ?
If not, updating may help.
Are you working on a Mac or Windows operating system?
Have you updated Lightroom to the current version that is 2015.3/
Do you have any external drive connected to your computer?
When was the last time it was working fine? Have you made any changes to Lightroom or your system recently?
I am using Windows 8 and was running the latest version of Lightroom and it was working fine up until 4 days ago.
- Restart the computer
- Reset preferences of Lightroom
- Update graphics card drivers
- Restart the computer again.