You can follow these steps to get it resolved.
1. Pull down the Server Name combo
2. Click on <browse for more...>
3. In Local Servers tab, expand the Database Engine element
4. Hopefully, you will find your server name here. Just select it and click OK. That's it
//If this doesn't resolve your problem
5. Sometimes it doesn't appears in the Local Servers tab and you may find it in the Network Servers tab
//if this doesn't help
6. Usually the server name is same as your computer name. Right click on the My Computer and go to properties where you can find the name of your computer. Type that as the server name.
//if this doesn't help
7. If you have installed SQLExpress then try adding "\SQLExpress" to the computer name while typing as server name.
Update
As the above solutions don't help, please try this
Go to SQL Server Configuration Manager > SQL Server Services > In Right Side window show all the servers which may stop. If you are using "SQLEXPRESS" then , Right click on SQL Server(SQLEXPRESS) and start
Try searching the solution in following links
Unable to connect to SQL Express "Error: 26-Error Locating Server/Instance Specified)[
^]
Solution: Error: 26 – Error Locating Server/Instance Specified[
^]
Hope, it helps :)
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