How do i get a good erection again?

i am asking this question because recently i tried to have sex with my girlfriend and i couldn't get hard for nothing. i see her beautiful naked body and she strokes me. but i still could not get hard at all. she tried everything on me and no luck. does anybody know any answers i could use? does anybody sell any over the counter drug i could take to get my erection back? i am also overweight and i hope that has nothing to do with my erection problem. i really do want to have sex with my girlfriend again and to give her the time of her life in bed. i do love her, but i could not get aroused at all. she does oral on me and nothing. please give me advice anybody in what pills i should take or should i see my doctor? i just don't know what to do. maybe i should get viagra or anything. is there any cheap medications i should take for my erection problems? i also have alot on my mind. eugene s