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Although this is not the exact model of your computer, the instructions are the same.
See the section Recovering the Internal Storage Drive, which begins on p. 60 of the User's Guide.
Satellite L450 Series User’s Guide
You were supposed to burn recovery discs, as it says on p. 68.
In meantime, try recovering according to the section Recovering to out-of-box state (recommended recovery method), which starts on p. 61.
If you have important files you neglected to back up, the procedure above will remove them.