Nothing will happen physically to the monitor. It'll just not accept the signal.
Can confirm, also overclocked a bunch of monitors. Works on most of them and if it doesnt, the screen will simply not work but won't get damaged. The CRU program usually used for this has a nice utility to reset to defaults and as other have said, when all else fails you can just uninstall the monitor via safe more.
If it is an option for you, I'd use a 2nd monitor (as your main monitor for the duration of testing) so that you can use that monitor to reset the monitor you are overclocking in case it won't show an image. Can save you a lot of hassle, if it's an option ofc.
so if I do from 60hz to 80hz-90hz I can be relax and don't worry about bad thinks?
if I do it have more electricity on the screen it isn't problem?
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I'd try to overclock in small steps only, and poweruse will probably not noticeably change, or at most in a very minor way.