You should discuss with him. Probably he got some difficulties to deal with.
Yup, first you should try to send him/her a private message, if he/she doesn't answer after 1 to 2 days, then you should seize the account. Sometimes they just can't pay back for the moment, add some interests, make a deal or something.
edit after reading last post: Wait a day and if she/he doesn't contact you just seize the account
first how can i pm him wdout knowing his email , new account , anything ! i,ll wait till tomorrow
that's why I edited Oo'
Also you should've asked his e-mail, skype or something to stay in touch with him.