How does one get a ROLL BACK from Lindens?

I own two plots of land in a private sim. Me and my neighbor both arrived yesterday to see over half our stuff including an entire arena of his and half my house and furniture gone.
I keep being told to get a "roll" back but of course cant reach any Lindens.

How do you get one? How hard is it to get? Will I lose all my stuff??

You will need to talk to the owner of the private sim. They can request a roll-back for you.

Unless the whole sim was similarly screwed over, getting a rollback for just two or three affected parcels in a sim is unlikely. A rollback is a sim-wide revert. Meaning everyone else in the sim also goes back to the date of the rollback, losing any prim items that were placed since that date. It can cause more damage than it heals, if done after any great deal of time has passed.

Is there any chance that the missing items were either returned to your lost and found by auto-return, or of the items being marked 'for sale' without you realizing it? Check your transaction log and see if any payments to you show up for those items - especially if they are payments for far less than the items were worth. There is a search bot in SL that lists any items that have the 'for sale' box checked, if you haven't opted out of their search. If the box had been checked and the default L$10 value was there, or if you had set the price to L$1 or L$0 to do a transfer to someone, a user of the search bot's website may have teleported to your parcel and 'bought' the items from you ar a stupidly low price.

I had a similar event happen on private land.... I had to get permission from all other owners on the SIm, who i have to say all willing agreed. The sim owner asked linden concierge team to do the roll back (it took about 3 days and a bit of badgering of the team) but eventually they were really helpful and the roll-back was fine - i got all my stuff back and gave the other landowners a small linden payment as thank-you.... hope you get it sorted.....