Upon further investigation, I discovered that the SQL Server (<instance name>) service was not started, and would not start.
The Application event logs had several 17207 and 17204 MSSQLSERVER errors which said that it could not open file c:\program files\microsoft sql server\mssql.1\mssql\data\mssqlsystemresource.mdf
The issue has occured because I do not have/use a SQL agent in my backup software to back the SQL databases up as they are (.ldf and .mdf). Instead I back all databases up using Maintainence plans in SQL Management Studio (.bak files). Therefore my <drive letter>\program files\microsoft sql server\mssql\data folder was empty with no mdf or ldf files. The SQL Server service requires some of these database files (such as master.mdf, model.mdf and mssqlsystemresource.mdf) to be in this location in order to be able to start the service.
Luckily, theres a copy of these required database files in c:\utilities\sql blank databases\sql 2005 data\2005 data. To fix, copy all files and paste into the <drive letter>\program files\microsoft sql server\mssql.1\mssql\data.
NOTE! If you use a drive letter other than C: for storing your databases you will also need to paste these files into C:\program files\microsoft sql server\mssql.1\mssql\data for the service to start!
Before being able to restore your databases from bak files you need to put the SQL Server service in maintainence mode. (In the properties of SQL Server service, add
–m into the Start parameters) Start the service.
Next, open SQL Management Studio, and restore the 'Master' system database (check the box to overwrite existing database)
Stop the SQL Server service. Remove the –m Start parameter then start the SQL Server service again.
Refresh SQL Management studio and you will see a list of your databases. You can now restore the databases one by one from the .bak files