windows7. It was working well but now a days when I choose
ubuntu at the start-up of the system it does not found any
Is there any way how to fix that?
Ubuntu will probably be looking in the wrong partition for the kernel.
I will edit this answer as you respond.
EDIT: This great answer here gives you more info on the WUBI installer. This could be part of the problem. Generally I wouldn't use the WUBI for anything other than to try Ubuntu. Anything goes wrong with your windows partition or install it can usually have serious problems with the Ubuntu install.