This could be a license checkout issue. Execute the following command to see if it gives an output of 1.
>> license('checkout','SIMULINK')
Or there may have been changes to the MATLAB path or settings. To restore the default path and clear cache, run the following commands in the command line
>>restoredefaultpath
>>rehash toolboxcache
>>savepath
Also delete the contents of the MATLAB prefdir which can be found by typing 'prefdir'.
You can also try checking the following page: Resolve Error: Undefined Function or Variable and the following MATLAB Answer: Why do I get the error "Undefined function or variable"?