What is the exact system model? What is the current OS installed? Are there any other issues with the machine?
As advised by the other user, the HDD has failed and needs to be replaced. If you are able to boot into windows, then manually backup any important available data to an external HDD. You would need the Dell OEM OS media to reinstall the OS on the HDD. If the system is under warranty, then you could also download the OS media online - http://dell.to/1JsMeJg
If the system is under warranty, write me a private conversation with the service tag and your contact details(Name and Email). If there is no warranty, If there is no warranty, then you could contact our team to get a quote for a paid service call - http://dell.to/1vnT6CQ
Keep us updated.