I use MySQL database for couple of weeks. Today I have worked in my database I saved it. Then I tried to open the database and saw - Index file is crashed. I checked backup copy it disappeared. Can someone suggest what can I do?

Check https://dev.mysql.com/doc/refman/5.7/en/myisam-repair.html

I'm not that familiar with mySQL, but this walks through repairing broken files within the database.

It does include instructions for repairing the index file,

Stage 2: Easy safe repair
First, try myisamchk -r -q tbl_name (-r -q means “quick recovery mode[/size]”[/size]). This attempts to repair the index file without touching the data file.

But I would read the entire article and follow the steps as listed.

Try to open database in another version of MySQL, in case it won't help, then you may use next command:

mysqladmin --defaults-file=/usr/local/mysql/my.cnf --user=root --password shutdown
mysqlbackup --defaults-file=/usr/local/mysql/my.cnf \
--backup-dir=/export/backups/full \
copy-back

When nothing helps when apply to Recovery Toolbox for MySQL, this solution has demo, you may try it on https://mysql.recoverytoolbox.com/