That's not enough information, I'm afraid. The playable range for celli goes down to C1 (that's two octaves below central C) en for contrabasses to C0. If your MIDI notes are in that range, you'll find that they sound well below violin range. Of course, the cello can also play in the lower violin range.
A related problem I had once, turned out to be a wrong track setting in my sequencer. It was a multitimbral instrument in Logic, I thought the track pointed to the cello channel in Aria player, but in reality it was the viola channel.