excel formulas are not working

My formulas are suddenly not working anymore. I copy and paste a formula and it shows the formula is there, however the value amount is wrong. It has copied the value amount from the previous cell. I can click on the formula and delete one number and then put back that same number and now it changes it to the correct value amount. Also, I created a formula for the sum of a few cells and the formula is correct. However the amount is not correct. I can highlight those cells and see the total at the bottom and it is not the total in the formula cell.