If you've been using a modified version of Spotify, but can no longer access it, there's really no security issue with your account: the company just disabled the unofficial apps due to an abnormal activity.
Spotify has sent out, via email, a warning to users of the modified iterations of the software, notifying them that the company is cracking down on the unauthorized versions of the app while keeping their accounts safe. That means affected users may still access their account, but need to download and install the official app on their device.
Free Spotify accounts are limited to the shuffle-only playback option and lack the feature which lets users download songs for offline listening. However, the modified Spotify versions had been allowing users to gain access to Premium-like services of the music streaming solution, such as the ad-free experience and the ability to customize the sequence of songs, among others.
The warning message states:
We detected abnormal activity on the app you are using so we have disabled it. Don’t worry – your Spotify account is safe.
To access your Spotify account, simply uninstall any unauthorized or modified version of Spotify and download and install the Spotify app from the official Google Play Store. If you need more help, please see our support article on Reinstalling Spotify.
If we detect repeated use of unauthorized apps in violation of our terms, we reserve all rights, including suspending or terminating your account.
It's worth noting that most of these illicit app versions come from open-source repositories like GitHub. Fortunately for Spotify, the software hub has removed many of these hacked Spotify modifications, including software called "Dogfood".
Spotify, which quietly filed for an IPO last December - and which revealed it had lost $1.5 billion in 2017 - , currently counts 159 million total subscribers, 88 million of which are on the free tier, while 71 million are paying subscribers. It's not immediately clear, though, how many accounts have been used in conjunction with the modified Spotify apps.