There is a way to get the file online, but as Arroyob stated your database will be inconsistent/unreliable. It is a technique that should only be used as a last resort or to extract data that would otherwize be lost.
If it is only one datafile, then it is possible that you can recover it with the logs available (if they are the most recent.)
It would probably be safer to just vary the file offline. The database will come up.
The best bet is tobite the bullet, restore your last cold backup and recover with the archive logs you have.
THere are other possibilities depending on your Oracle release.
Joseph R.P. Maloney, CSP,CDP,CCP
'The answer is 42'