It sounds like you are using the mobile version. Don't know what the problem with it is, but it seems to do that regularly.
I would scroll down to the bottom of the left column and hit the link for Basic HTML. IMO, its a much better interface, anyway. Make a bookmark and delete the default GMail icon.
Another thing you could try is to go to the browser options, Privacy & Security then Clear Cache and Clear Local Files. I have been using the basic html version as I have had to do this a couple of times with the mobile version of Gmail. I get similar compatibility issues with both the mobile versions and desktop versions of Gmail and Google+ which often just crash the browser. My suspicion is that it is the Playbook's javascript implementation.
Another thing you could try is to go to the browser options, Privacy & Security then Clear Cache and Clear Local Files. I have been using the basic html version as I have had to do this a couple of times with the mobile version of Gmail. I get similar compatibility issues with both the mobile versions and desktop versions of Gmail and Google+ which often just crash the browser. My suspicion is that it is the Playbook's javascript implementation.
ONLY clearing local files WILL solve the problem. There is no need to clear the cache.
i think google doesn't like us