Windows 10: No Operating System Found (Please Help)

From the T520 users manual:
Make sure the computer is turned off. Repeatedly press and release the F11 key while turning the computer on. When you hear beeps or see a logo screen release the F11 key.

https://www.manualowl.com/m/Lenovo/T...12599?page=132

 

No I meant I don't know how to factory reset my Dell Latitude D810. It's my second laptop but it's old and slow, like really slow.

 

Have you tried doing a defrag and running CCleaner to see if that improves it, you might find that will help you a bit

 

The defrag didn't work,but the CCleaner did help a little,thanks.

 

At least it is a start, defraggler will need a bit of time to work if bad can take up to 2-3 hours, then after run CCleaner again also go through the control panel and remove any programs you don't use
Keep us updated then when solved mark it so and it might help another member with the same problem.

 

I'll be trying the defrag again, hopefully it works this time