When a person creates a trust, they transfer their assets to a trust.
They may be the trustee during their lifetime, but when they die a successor trustee takes over. Upon their death, the trustee is obligated to send trust accountings to all beneficiaries, and provide them with a list of trust assets upon request. Sometimes beneficiaries do not receive the accountings, and the trustee will not communicate with them or even give them a copy of the trust. If the trustee continues to refuse to cooperate, a legal action can be filed against the trustee to demand an accounting.
Sometimes beneficiaries believe that a trustee is not paying them funds that they are entitled to, or misappropriating funds.
As with a personal representative, a trustee is a fiduciary, with the highest level of trust to act for the benefit of the beneficiaries. If a trustee is removing funds, or failing to pay income to beneficiaries, or otherwise mismanaging the trust, a beneficiary can file a lawsuit against the trustee.
The lawsuit may be to compel the trustee to make payments, or it could be to remove the trustee, or both.
Reformation of trust:
Sometimes a trust has become ineffectual. That might be the case if the trust has gotten so low on funds that it can no longer effectively operate, or if the all trustees have died or become incompetent.
The beneficiaries may have ceased receiving income. At that point, the beneficiary can file an action to reform the trust; to appoint a new trustee, or change the payment terms, or even to terminate the trust.