Whenever I go to toilet and apply pressure on the stomach there is sudden backward movement of penis and all the stuff comes up & I cannot empty it. Please help me.

Stop apply pressure to your stomach then.

Men who have trouble emptying their bladder should have their prostate checked. Enlargement is extremely common, and the prostate presses on the ureters, blocking them. Most of it is benign, called BPH. You can google BPH for tons of information.