Can someone explain to me what happened?

[RazorHound]

I was in the middle of a two hour long rescue mission, I had just moved the rescue-y to the rescuer's ship. I was in the middle of a turn when I accidentally pressed Tab, switching my view to Kerbin. I switched to the rescue-y's ship and then switch back to mine to bring back the focus, but when I switched back, both of the kerbals were deleted and I had no control over the ship, and my 2 hour long project burned up in the atmosphere?

What happened? Did I do something wrong? 

Thanks have a good one, ladies and gents

[Starman4308]

So, to be clear: you were in reentry at the time?

Also, backspace returns you to focusing on the ship, and you can back-track with shift-tab (caution: shift also used for throttle).

If you were in reentry, it's entirely possible the vessel just got deleted when they got outside physics range of the current vessel. If it was just the Kerbals inside the vessel... that's basically the Kraken telling you you can't have nice things.

[The Space Dino]

2 hour long rescue mission?

Were you in low Kerbin orbit?

When I did a rescue mission it only took me 1 hour, and it was my first try.

[Sharpy]

One more nasty thing. At normal time wrap (not phys-) - even mild 5x, '>>' - collisions between near craft/objects don't work. Two ships can  literally phase through each other. The moment you come out of warp, physics kicks in, and current 'conflicts' get resolved - sometimes in an explosive manner. If your kerbals happened to 'phase' into the shift, and you came out of time warp - kerbals are softer than ship parts; Poof-Poof, gone!