what job does your sim have ?
a couple of things to try first simplest one is get your sim to phone up and request 1 day off work it seems to usually acknowledge and stop your sim missing work on a day off
or your sim can possibley have a corrupt work outfit send sim to plan outfit-career if the sim has more than on eoutfit delete the extra if the sim has one outfit then choose a new outfit
have you tried the resetsim cheat ?
ctrl+shift+c hold all 3 together in the menu bar that appears at the top of the screen type in resetsim * (hit enter ) this will reset all sims inculdeing your own to reset one sim ctrl+shift+c hold all 3 together in the menu bar that appears at the top of the screen type resetsim first name last (of your sim ) hit enter