Cub cadet XT1 42In riding mower. Was mowing and blades suddenly quit mowing. Manual PTO lever is very loose. Doesnt do anything. Checked the belt and it appears ok.

SOURCE: I have a Cub Cadet 1250. The PTO suddenly stopped

Look at www.cubcadet.com. There under parts you should find an IPL for your mower. You will need the model and serial number from under the seat to get the information. Look under the electrical for PTO.
HTH
Good Day.

SOURCE: I have a John Deere 102 riding mower with a manual

Check the belt its not tight, its loose or not fitted right.------ May be you put the belt loose or not properly.------ I agree belt is new, the belt is not the problem but its placement is problem.its coming out of its pulleys.Recheck that belt is intact and in-place and not getting loose.-------- Also recheck the connection of pto switch and seat switch.Remove the switch one by one and by pass it with a jumper wire and see if the blades getting engaged.------- ----------
Also check the bolt that holds the idler arm to the deck ,that bolt gets worn out causing the the arm to move up and down. ---------- This will help.Thanks.Helpmech.