You just install a new instance of magento on a new server, for testing by example and you are unable to login. Password recovery send you a new password but it doesn’t work. This is probably a domain cookie issue.
After installing a new instance, you change in the database, table core_config_data, these 2 entry : web/unsecure/base_url and web/secure/base_url. Don’t forget to change this one too : web/cookie/cookie_domain and web/cookie/cookie_path