[Ultimaker 2+ extended: crossbars have come loose]

The crossbars on which the print head sits ride on pairs of brackets, each bracket attached to its drive belt.

 

All of a sudden, both of the crossbars are loose in their brackets and can become entirely disengaged, which obviously causes a print failure.

 

Haven't seen any mention of this. Has it happened to other people? Any recommendations for a fix?

[My prints are not sticking to the glass anymore]

OK, here it gets interesting. Here's the start of a brim for three circular pieces.

Bear in mind that's immediately after bed levelling! Hairspray on the glass, but it makes no difference - that seems very obviously like too much gap between bed and nozzle.

Is it plausible that the buildplate is somehow slipping down after it's levelled?

[My prints are not sticking to the glass anymore]

A similar thing's happened to me in the last few days. No change to software or to printer, but suddenly nothing's sticking any more. Cleaned the plate, re-levelled (four or five times), tried glue stick and hair spray even though I rarely needed them before… the first layer looks fine, but by the time I get a few cm up the print breaks free from the surface. Help!