Hmmn. Sounds like the mirror's sticking to me - as in; the viewfinder only showing a narrow band of image, and live view not working. The kind of motor (really an actuator, and they can stick at either end of their movement, which would explain the black viewfinder) that's used for mirrors needs a lot of power and will stick at a point of resistance such as when they change direction, so if your battery is failing it won't be able to give the power needed. I'm pretty certain that changing the battery - for either AAs through an adaptor or another lithium - will fix it.
My hobby before photography was electric powered radio-controlled planes - aerial videography is great fun - so I had to learn a fair bit about lithium batteries and there's about a 1-in-10 chance that an untested lithium battery will quickly lose the ability to hold a useful charge. The batteries I was using were lithium polymer which sometimes explode if they go wrong; cameras use lithium ion batteries which are much safer, so they're not tested nearly as much, which includes how well they can hold a charge under load. As I said, actuators (basically electro-magnets) like the ones that are used for mirrors in DSLRs need a lot of power, so if the battery can't give it, the mirror won't work properly. The battery meter in your camera needs almost no power so it will still show as full even if there's only a little power in it.
I've found that ebay batteries are as good as any, so maybe think about getting one of these to see if the battery's the problem
http://www.ebay.co.uk/itm/D-Li109-Battery-for-Pentax-K-R-KR-K2-K-2-Digital-C...item3cbe47d16f. If you're not happy with putting a cheap battery inside your expensive camera you could try one of these.
http://www.ebay.co.uk/itm/Pro-AA-Battery-Holder-Box-For-Pentax-KR-K-R-DSLR-S...item3cbe47d16f.
Oh, another thought, sometimes battery chargers go wrong... Whatever, I'm sure the camera shop will have another battery to try in your camera and that will hopefully solve your problems.
Shame to not have your camera for your birthday though, I hope you had a good time anyway.
P.s. - I dropped and broke nearly 4,000 euros worth of camera on my birthday, so I know how it feels
Last edited by wildweasel; 02-17-2013 at 07:13 PM.
Reason: Messed up link.