My acer one d250-1165 cant start why?

Any error message?

To backup your important files to an external drive or to another computer through the network, you could boot on an Ubuntu USB pendrive. Alternatively, you could take out the hard disk and connect it to another computer.

Then you could restore the factory default setup if the restoration mechanism is supported on your model (you have the restore disks, or the computer has the hidden restore partition with the factory setup system image).