Hello, I've got an inspiron 17 5000 series i3 laptop. I have changed my hard drive with an SSD. Sometime laptop fails to start, but when it does there is no internet connection and i cannot even open a file, i can't use my programs, cant use the start button and the taskbar and everything seem to not work properly. Sometimes a message appears saying "Uknown Hard Error". How can i resolve this? Thanks in Advance
Start with a diagnostic on the SSD - while you can try the Dell diagnostics (F12 at powerup), it'd be better to use the diagnostic supplied by the manufacturer of the SSD.
Why are you changing your settings between Legacy and UEFI on an existing Windows installation? If you need to change that, it should only be done before performing a clean Windows install, unless you're specifically trying to use the MBR2GPT utility in Windows 10. Otherwise, leave it alone. Anyhow, it sounds like your Windows 10 installation tried to install an upgrade to a new release, and that may not have worked properly. Rebooting multiple times to restore to the previous version is normal behavior, so if you haven't already, you might want to just wait for a few reboots. It's possible that after installing the new release it detected that some application incompatibility would prevent it from booting and it's now falling back to the previous release automatically.
But if this is something you would like better protection against in the future (especially if your system does NOT automatically recover), you may want to consider performing image backups of your system. Macrium Reflect Free is a popular option, and as the name suggests, it's free. If you decide to use that as a regular backup mechanism, the paid version may have features that you would like to have, but at the very least you could use the free version to capture image backups immediately before major Windows changes or application installations so that you can always recover your entire system if needed.