I have reset my iPhone, now many contacts are lost, though I synced them through iCloud. Are they still in an old backup?
Not sure what happened but you should be able to restore your iPhone from within iTunes. Depending on when the backup was made that should have everything that was on your iPhone.

If you have no backups in iTunes that work you can check Time Machine for other backups:

/Library/Application Support/MobileSync/Backup/


Restore your more recent backup to your local HD and then Option-Click on the restore from backup button in iTunes and select the file you just recovered.

I am doing some of this from memory but that is the general idea.