After I instaled extension in typo 3 version 4.5 I can't log in to backend, my page is blank and after I try to go on http://mydomain/typo3/install/index.php I get this message :
The Install Tool is locked.
Fix: Create a file typo3conf/ENABLE_INSTALL_TOOL
This file may simply be empty.
For security reasons, it is highly recommended to rename
or delete the file after the operation is finished.
If the file is older than 1 hour TYPO3 has automatically
deleted it, so it needs to be created again.
Installing any extensions in such an old TYPO3 probably will kill your installation, as any modern extension is incompatible to the old sources.
Modern extensions have namespaces, autoloader, use API classes and functions unknown to the old installation, maybe even use PHP7 features unknown to the old PHP which is needed to run those old TYPO3.
now for your cleanup:
For entering the instal-tool do as the message indicates:
create/update that file and then you can enter the install tool.
but in 4.5 you will not get much help to get the system running again.
you must disable the malicious extension:
in the file typo3conf/localconf.php you need to delete the extension key from the list of installed (active) extension. For this you can use any editor. One is build in the InstallTool.
After that you need to remove the cached version of the localconf.php: remove the temp* files in the folder typo3conf/.
Now your system should work again (but still might be unclean).
you should remove the extension (typo3conf/ext/extensionkey).
Maybe there are new tables and fields in the database. Therefore do a database compare in the InstallTool.