Sublime Forum

Undefined control sequence


I am relatively new to sublime text so maybe the problem I have is simple to solve.
Generally everything was working fine writing the Latex document until I closed and reopened sublime text.
Now I get undefined control sequence error for the code that compiled just fine 30 seconds ago.
For example in this code:


I get error “Undefined control sequence. [&\mu=\sum\limit]”
What can the problem be?