[AC Question]

Hello, im in the black now. At first i thought it was my Sd card fault, my files corrupted with "u" name folder. And the files goes to LOST.DIR. I thought its not big deal, and i backed up the file before so i dont mind if i have to format my sd card. But i cant format it, the files still there.
And i bought a new one, but it still happening later so i have to keep formatting my sd card. And this time i can format it every time it happens (and im tired of losing important data i havent back up).
But then, last night i didnt find the same folder and nothing goes to LOST.DIR folder.
I realize something wrong when i cant check any apps size (when i check the app size it stuck at "computing"), i cant install anything, i cant even format my SD card(it stuck at unmounting). What is happening with my phone?? Im not expert and i use samsung core 2(i know its old) and T.T sorry for bad english