You can download the version of LR you were running from this link.
If you have the newest version, LR CC, then you have a subscription and you will first need to install the Adobe Creative Cloud desktop app then download and install LR from that.
As to your pictures if you didn't have a back copy of them they are gone.
That is unfortunate. Did you have a backup? Getting Lightroom back is easy as you just download it again from Adobe either direct or through the creative cloud thing, but if you want to get your pictures back you would need to restore from a backup.