will hosting provider have access to my dedicated server?

Yes, they will have access to your server. If virtual, they have access through the virtualization console or container root. If physical, IPMI and out-of-band management provide access. They may have access to your backups. They definitely have access to your disks...

In 2000 Microsoft published something very smart and is still (mostly) relevant today. The 10 Immutable Laws of Security http://technet.microsoft.com/library/cc722487.aspx

Rule number three is "If the bad guy has physical access to your computer it's not your computer anymore."

Fact is you should consider any computer you don't have COMPLETE physical and technical control over a potential target for compromise. Here's a link to think on: http://felipeferreira.net/?p=1259

Depends on the provider.
Usually if you change the password they don't have access anymore.

However: They have physical access.
They can just take out a disk from your RAID1 and have all your data.
They can reboot your server and reset your password, or boot from a CD and read it all, ...