You DO understand that any people on here ONLY have what you typed to go by. They don't know you, your level of analytics, your technical level, etc. - thus offering the obvious solution (for starters), like triage in a hospital.
When you attempt to download the app, is it an INSTALL, or an UPDATE ? (I know you mentioned update, but you said you don't have the app)
I'm gonna offer another solution - try downloading SYSAPP Pusher, configure it appropriately & check your apps ... 'cause frankly, I dunno - more info might help.
& you may be stuck doing a HARD RESET, but simply as a last resort (I assume you know the ramifications of a hard reset).
one other thing, on my Lumia 1520, I can do 2 different soft resets (I don't know the difference, but they behave differently)
- hold POWER in, until vibrate (~ 10 seconds), then release immediately, may simply be another method of restart
- or hold POWER & VOLUME DOWN, until vibrate (~ 10 seconds, little less than the first method), then release immediately