Once you buy Minecraft (from the official website), you personally have permission to play it, forever. On any computer (at least, any PC for the PC version). You can log in to https://minecraft.net/ and download it, and install it, as many times as you like.
There's other things we could try, but uninstalling-reinstalling does fix a lot of problems like this.
The "unknown error has occurred" is a Windows error, rather than a Minecraft error. Unfortunately, a lot of things can cause that error. It could be some other software running on your computer - such as an antivirus program. So you could try closing down all other applications.
It could be a "permissions" problem - that your user account is not an admin. Maybe you could change it to be one, to try that.
Otherwise, please do try it again, and get the exact error message - or take a screen print.
But anyone offering support is likely, at some point, to say "try uninstalling it and reinstalling it".
As long as you know your minecraft username/email and password, then you won't have any problems with the payment side of things.
unknown error is shown when showing the crash report
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Op in #minecrafthelp, JIRA Helper in bugs.mojang.com, Chat moderator in Minecraft Forums, Twitch/Mixer mod
After i hit the play button on the launcher, it takes forever to load. Then ends up crashing before i get into the game
It said "An unknown error has occur causing the game to crash". I think so at least
It has worked before, i was working right before it started crashing
I haven't tried uninstalling yet, mostly because i don't know how. Cause I'm worried I'm going to have pay for it again
And it is totally vanilla, no mods
unknown error is shown when showing the crash report
Op in #minecrafthelp, JIRA Helper in bugs.mojang.com, Chat moderator in Minecraft Forums, Twitch/Mixer mod
How to get a dxdiag
If I helped you, dont forget to click the thanks arrow!
Please follow the instructions on this page: http://hopper.minecraft.net/help/guides/minecraft-logs/
Op in #minecrafthelp, JIRA Helper in bugs.mojang.com, Chat moderator in Minecraft Forums, Twitch/Mixer mod
How to get a dxdiag
If I helped you, dont forget to click the thanks arrow!