Why are there no eggs?

Thanks, but the male is still around. The other day, both of them spent a few moments in the nest, and one of them gently poked the other one a couple of times and then left the nest.

Things are going well now. I saw four eggs today.

Good news, thanks for letting us know! Please update if this is a successful nest!

Very strange update.
The bird laid only 5 eggs and incubated them for about 14 days and all hatched over two days, but then they all died after three days and the parents disappeared and the bird box only contained the corpse of one chick and a few flies.
Then I noticed that the corpse was gone and the nest was empty and the coal tit began sleeping in the nest at night.
This has gone on for about two weeks and the bird now spends a lot of time hanging around in the bird box during the day as well as sleeping in it at night.
I saw both parents in there one day for a moment, but only the mother since then.
There are no eggs in the nest but the mother keeps rearranging it and settling in like when she was nesting.
What’s going on? Are they going to have a second set of eggs soon?
If not, I think the bird box needs cleaning since I sometimes notice a fly in there as well as the bird.
Maybe the traumatised bird is just going on instinct after all the chicks died.
I’m tempted to leave the bird box nest to see if she adopts it as a permanent roost but I’m worried about it causing disease or parasites if it’s unclean.

Nest boxes shouldn't be touched until end of breading season..usually end of August regardless of what's in them am afraid ..its the law

A useful link below

www.rspb.org.uk/.../