i have a 500 gb hard drive partitioned for files and time machine but when i connect it to my mac only the time machine one shows up

well as the title says i have a 500gb hard drive with 2 partitions one with time machine and one for files it worked perefectly until yesterday only the time machine partition appeared in finder, i checked disk utility and both partitions appeared but the one for files appeared in grey and when i press mount an error comes up.