If Steam does not start, it is likely that his process is just stuck in the computer's memory. The hung process does not start a new program file. To remove unnecessary task, press the combination of buttons Ctrl, Alt and Del computer keyboard. In the appeared menu choose "task Manager".
You will see a list of processes currently running on the computer. Among them, find the name of Steam.exe and right click on it. In the context menu click "Finish" to remove the process from memory. If shutting down did not help, click again the right mouse button and select "End process tree". Once the operation is complete, close the window "task Manager" and try running Steam.
If this procedure doesn't help, there are alternatives. Go into the directory program, which can be found in the menu "start" - "Computer" - "Local disk C:" Program Files – Steam. Delete the files tier0_s64.dll and tier0_s.dll, and then double-click on Steam.exe to execute run the program.
If the above methods do not help, try to delete all files in Steam directory except Steam.exe and steamapps. After removing double click Steam.exe to check the performance of the program.
In the absence of reaction on start up you can reinstall the client service, however you will need to carry out cleaning of system from unnecessary files manually. Run CCleaner utility on your computer. If the program is not installed, download it from the official developer site and perform the installation according to the prompts on the screen.
Go to official Steam website and download the latest version of the client from the section "Install Steam". After downloading the program, run the downloaded file and install the application. Then use the created shortcut on the desktop to launch the application.