One day randomly my mercedes R350 2006 Key fob stopped working, it won't turn over or start the car, buttons don't work on it either although the red light flashes when a lock/unlock button is pressed, very frustrated I tried changing battery and checked fuses they all look good, had no luck, any ideas what it could be?

any advice much appreciated thank you

ps: i don't have a spare key to test it, and new key costs $350

The key turns, but won't start, no clicking or anything
Convenience functions don't work nor the buttons on the key fob it's like the key fob isn't even synced to it

Without wanting to provide bad advice, perhaps a battery disconnect/reconnect may reset the connection?
The reason is my friends 2005 VW Golf exhibited the same problems and after following a dozen instructions that didn't work, this did.
However, I don't know what items you have to also reset on an R350 after disconnecting a battery... I know on my w203 there's a few things like windows, ESP, that needs to be done. .. I imagine it would be the same here.
Best of luck

I like JonMac's suggestion, as some electronic issues resolve themselves after a full power off cycle. His cautions are pertinent as well. Just one last possibility, which I literally have done myself... Do you happen to have two mercedes cars and are you possibly using the key of the one to open/use/start the other? I did this a few weeks ago when our new (used) merc had arrived and our old/outgoing was still here. Pretty dumb of me :-)