I have lost the thumbnails in bridge how do I get them back

That is very less info to guess from.

Can you provide more details about OS, and exact version of Bridge.

Was it working before and did it happen suddenly.

Basically Bridge is a browser and it shows the contents of the folder you have pointed it to. Check on your computer if there are still files on the folder that does not show thumbnails.

Can you provide a screenshot of what you are looking at to give us a better insight

If you do not see thumbnails what do you see?  Have you looked in View and made sure Thumbnals are selected?

Run the Apple Utility disk and permission repair.  See if that helps. 

Did you try to reset the preferences?  Hold down Option key and start Bridge.  You should get a reset window with 3 options .   Choose all 3.

You can upload a screen shot by clicking on the camera icon in the reply box.

I will try the reset first

If that does not fix it try this:

Quit Bridge and go to your user library (by default hidden per OSX10.7 - use menu Go and press option to reveal the library)

Inhere choose Caches / Adobe / Bridge CS6. In this folder are 2 items. a plug in cache and a folder called cache. Drag both items manual outside the library, preferable in the trash.

Then in the same user library go to the preferences folder and find and delete the plist file called : "com.adobe.bridge5.plist"

All deleted files will replace themselves with new and fresh files once restarted Bridge.

Hod down option key while restarting and (again) reset the preferences (only preferences is enough).

Then try again. All has been set to factory default but custom workspaces and keyword list etc should be available. Custom workspaces probably need to be reset and your custom preferences have also gone to default, so you have to change them again to default wishes.

And the cache needs to rebuild itself, this takes some time