Re: my phone is completely blocked since downloading an app
Try soft resetting the phone with the button combo, if that fails proceed to a hard reset. If both fail then we still have the windows phone recovery tool, but this will wipe all device data, once you gone through the process and are setting up your phone it will ask you if you want to restore from a backup. It's quite obvious that you should pick a backup that has a date set before you installed the app.
I should point out that I've never seen an app do this, so a soft reset might help
Soft reset (restarts the phone) : Volume Down + Power button (for 10 seconds, until you feel a vibrate)
Hard reset (erases all data on the phone) : Do a soft reset, but don't release the Volume Down button (you should see a large exclamation point) press (in order) Volume Up, Volume Down, Power, Volume Down and that will do a hard reset.