[blykins]

First thing to check on something like this where it goes into gear easily in 2nd, 4th, etc., is the shifter ring/boot. It could have moved, causing the shifter to be prohibited in its movement.

Also, if you have an aftermarket shifter, check that the stops are set correctly.

If you were having trouble in all gears, I would suggest checking the clutch adjustment and/or bleeding the master/slave cylinders.