Did you drain the oil to look for metal?
Do you have stands? If so put it on the back stand and try to shift gears without the bike running, use your hand to move the back tire, all you are trying to do is get it to go into any other gear.
Did it make any noises before this happened?
How did it shift before? Was it smooth, clunky, missing the odd shift, needing extra effort to move the shift lever?
Check your front sprocket to make sure it is secure.