Server 2012 R2 Disk Troubleshoot

My Windows Server 2012 R2 box has begun throwing disk-related error messages and overall performance has slowed to a crawl. I am not sure whether the root cause is hardware (drive, RAID controller, cabling)... (Budget: ₹1500 - ₹12500 INR, Jobs: Active Directory, Data Recovery, Documentation, Network Administration, Powershell, Server, Technical Support, Windows Server)