1) First thing is why did you change your mac address to begin with?
2) Some people think that websites on Internet can track users using MAC address and so they must change it! Well, your MAC address wont go further than the first router the IP packet gets to. Its because, MAC address is used as unique identifier at data link layer (OSI layer 2), like for example your local Ethernet, while the IP packet being forwarded over Internet is at network level (OSI layer 3) and uses IP Address as a global unique identifier (which websites can see and track). And when an Ethernet frame reaches a router, the router's MAC address is used to send the frame forward. So, clearly this is not a reason to change MAC address
3) If you are in a public place it is far better to use a VPN
4) If your ISP uses MAC address to identify or authenticate your Internet connection. So in case your network card goes boom and breaks, the new card you replace it with will have different MAC address and so the Internet wont work. So changing the MAC address to old network adapter is the quickest fix instead of telling your ISP to register your new MAC address which may take lot of time. This is why you should always right down the MAC address that is working before you make any changes so you can change it back if necessary.