Without better info, it sounds like your boot sector is messed up or you have bad system files.
Either way, it will require a reinstall of the OS. You may be able to do a repair install. But first you need to download and create a Windows USB flash drive. Then boot from that and follow directions.
A repair install will not delete your files. A clean install will delete everything. If you need to recover your files (no backups) then you must remove the drive and connect it to another computer using an external drive enclosure. However, it may not register on the other computer if the boot MBR is bad.