Hii, Welcome to the Forum !
Here are all important links:
http://lmms.io/forum/viewtopic.php?f=1&t=4740
-A few rules and useful forum instructions
I think you may have pushed it outside view, try looking to the rightpiano roll : from a certain moment, it does not open anymore
...
If thats not it, please post your:
* OS (+service-pack-version)
* lmms version
* hardware specification (ram & soundcard is the most important)
* a screenshots of error-state -if any oddness appear