Using Project 2013, I have a massive project on the go, and spent ages typing in the actual hours done on various tasks at various dates from employees' time sheets. Today I marked some tasks as 100% complete, so I can compare % of tasks completed and hours used against estimate, and it appears to have wiped all my individual entries of hours worked for those tasks, and replaced the total actual work figure with the baseline hours I originally put in.

How can I get back the actual hours worked across the calendar, preferably without wasting days manually entering them?

What should I do in future to stop this monumentally annoying thing happening again?