No first gear

Selector pawl not free and centralised (with a little clearance ) in the spring?.

If the box has been apart it may not have been indexed properly, Mick Hemmings video makes it clear and simple to fix.
Dan 

If the  gears can be selected going up to fourth but not coming down then the problem could possibly be a broken return spring. This is the chunky spring that sits under the pawl spring mentioned above by Robert.  Photo below.
Otherwise Dan's suggestion would be my next line of investigation..

The return spring as shown in Phils reply can have a "dead stick" failure crack that it almost invisible untill its out for inspection.

Can in suggest you invest in the clubs gearbox overhaul video by Mick hemmings, its explains it all very simply. Personally i dont like gasket cement, unless its wellseal! 
dan 

I 2nd Dan's comments having just dug into my gear box with Mick's helpful video. 

The video's, parts books and manuals are worth their weight in gold, I'd wish more owners / garages use them. 

If you have this sort of issue on the 'side of the road' or are just a 'skin flint' who wants to save gear box oil, then throwing the bike on its Left hand side against the garage wall keeps all the oil in the gear box!

... it suggests the timing is out by 1 tooth. If you go through the box to top there is likely to be quite a big gap there. No real way of fixing it other than taking the inner cover off as far as I know.