problem in linux mint installation

hi... i had window 7 and ubuntu on the hard disk of my laptop. I needed to erase ubuntu and install linux mint in place of it. so while installing mint from the bootable usb i chose the option of erase ubuntu and install linux mint in place of that. bt i dnt know what went wrong the istallataion was unsuccessful now when i boot my laptop error message comes that unkwown file system : grub rescue> . Please suggest me some way to fix the problem and do i require to format the hard disk to reinstall linux mint over the whole hard disk.

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