Hi Peter, Sorry to hear about the nest-box being dislodged; I would be surprised if Blue Tits were fledging at this time so it may be that the hatchlings were predated. Corvids, including magpies will take young chicks, eggs, etc, Although it's not totally impossible for BT's to have a 2nd brood if they've gone through the whole process already, it is unlikely I would suggest as it's an exhausting process for them; they do tend to time their breeding to coincide with the availability of green caterpillars (oak moth) which is the predominant food for the nestlings/fledglings although with this nest failing fairly early on the female may have started laying a 2nd clutch again elsewhere, I'm not sure they would return to the same box this year; I think most blue tits tend to fledge late May/early June time.
The other possibility of box dislodgement could be cats if they can access the box from fence or branch, etc., or squirrels if you have them. We have squirrels so try to secure the box with an extra galvanised nail (our nest boxes are on our oak trees) either side of the box to stop them tilting or dislodging; hindsight is a wonderful thing but its always best to try and ensure boxes are very secure when you mount them and can't move, boxes preferably in place before January time if possible as birds will start inspecting likely nesting places very early on in the year and will use the boxes to roost in them over wintertime.
Good luck and hope you get to see some lovely fledglings in due course.