Just charge the phone with the wall charger, if you bought the N900 as new, you should have the manuals - please do read them, atleast the first steps... The charging process is already described in this thread somewhat. After a full charge, just try to start it up.
The phone probably did not have enough power at your first sync, and your comp did not provide enough to charge it, so overnight it emptied the battery. This might have messed up something in your phone, but I hope not.
Also, the sooner you update to PR1.3 the better - I suggest that you try to flash the phone even before you install anything else. 1.3 has improved lots, usability and google sync support is improved. Also Ovi Suite support is better.