Turn on your PC, and when you see this GRUB menu:-
Select the 'default' kernel (the top one), and rather than pressing enter, press E
You will be presented with a screen SIMILAR to this:
Press DOWN ARROW
until you get to the line that starts with:
and press the END
key to position the cursor at the end of the that line... it *usually* ends with “quiet splash”.
Now use the backspace key to remove the words "quiet splash"
(if present, also remove the "$vt_handoff" parameter)
Now hit Ctrl+X
It should now display what's happening during the boot process .. does it give any clues where the boot is stalling ?
(eg.. any errors, or the last text that's displayed before it stalls)