You face such problem after upgrading your wordpress or installing a new theme, all pages of your wordpress start showing blank page and there is no other way to access your wp-admin panel to change or manage the theme you installed.
You can resolve it by renaming the currently installed theme from FTP, which will force wordpress to reset your blog to default theme. By doing this you will be able to access your wp-admin panel to edit / change your theme.
Navigate to wp-contents/themes directory via FTP and rename the theme you installed or the active theme due to which your blog start showing blank page. For example "twentythirteen" to "twentythirteen-bk".
Hi Gaurav, Welcome to MilesWeb Community. Thank you for sharing & it should help bloggers who face such issues.