Try some of these “fixes” below if:
the printer does not heat up and never starts printing but the progress bar continues to fill
the printer heats up and never starts printing
the printer heats up and never starts printing but the progress bar continues to fill
the printer stops printing but the temps remain and the progress bar continues to fill
the printer goes through the starting script and then goes where it shouldn't (this could happen for other reasons also such as incorrect slicer settings)
Best long term fix is to purchase a new microSD card that works with the printer. I have had good luck with this Kingston 8 GB microSDHC Class 4 Flash Memory Card SDC4/8GB from Amazon. See also this page microSD Card.
If printing via microSD card:
Make sure you have sliced the 3D model file which will generate a .gcode or maybe .gc file. The printer will not print 3D model file, for example .obj and .stl
The included microSD cards are known to get corrupted over time. If you open the file from the microSD card in a text editor such as
Notepad++ or
Sublime Text you may find a lot of U's further down in the gcode file.
Format the microSD card. If using Windows uncheck the Quick Format box. It is best to perform a full format to scan the card for bad sectors.
If using Cura to slice make sure you save the gcode file from Cura to the hard drive and then from the hard drive to the microSD card.
Try other microSD cards.
Try any card up to 32GB (yes some 32GB cards will work with the MP Select Mini V2 and the MP Mini Delta). FAT16, and FAT32 both work. Also exFAT works for some. The printer is picky about which cards is will read so if you have multiple microSD cards try them all.
You may also want to try formatting the larger cards such as 16GB and 32GB with different allocation unit/block sizes.